A conventional magnetic tape cartridge consists of one or more rotatable tape reels for moving the magnetic tape. Each of the reels typically has flanges which guide and shield the tape from a stationary outer protective shell. Cartridges of this type usually contain a brake and a clutch. The brake prevents the reels from turning and loosening the tape when it is handled outside of tape drive. The clutch is provided for engaging the drive motor to turn the tape reels.
After providing for clearances, tolerances, and clutch and brake actuation, these cartridges are typically twice as thick as the width of the tape they contain. For example, a cartridge which contains one-half inch wide tape is approximately one inch thick. This geometric constraint reduces the volumetric efficiency of cartridges, which is defined as the volume of tape divided by the minimum rectangular envelope of the cartridge. Cartridges with a single reel have a volumetric efficiency of about 20%, whereas cartridges with more than one reel have a volumetric efficiency of about 8%.